About Oscarsborg
Just a few minutes by ferry from the town of Drøbak, where the Oslo
Fjord is at its narrowest, you find Oscarsborg. An idyllic coastal gem
where stately fortifications and underground tunnels accompany the
barracks turned hotel, and where biblically named cannons set the mood
for a 'banging' visit with a historical buzz.
Oscarsborg Fortress (Norwegian: Oscarsborg festning) is a coastal fortress in the Oslofjord, close to the town of Drøbak in Akershus County, Norway. The best known part is situated on two small islets: Nordre Kaholmen and Søndre Kaholmen. In addition, the main artillery batteries were on the island Håøya and smaller batteries were on the mainland to the west and east of the fjord.
The fortress is best known for sinking the German heavy cruiser Blücher on 9 April 1940. The fortress was military territory until 2003 when it was made into a publicly available museum and resort. In 2014, Oscarsborg Fortress was given protected status.
For more information, please see the Wikipedia article or the National Library (bad Google translate). It has been established a cultural heritage trail for the fortess (See picture).